struct Y { Y(int); };
struct A { operator int(); };
Y y1 = A(); // error: A::operator int() is not a candidate
struct X { };
struct B { operator X(); };
B b;
X x({b}); // error: B::operator X() is not a candidate
— end example
class B;
class A { A (B&);};
class B { operator A (); };
class C { C (B&); };
void f(A) { }
void f(C) { }
B b;
f(b); // ill-formed: ambiguous because there is a conversion b → C (via constructor)
// and an (ambiguous) conversion b → A (via constructor or conversion function)
void f(B) { }
f(b); // OK, unambiguous
— end exampleConversion | Category | Rank | Subclause |
No conversions required | Identity | ||
Lvalue-to-rvalue conversion | |||
Array-to-pointer conversion | Lvalue Transformation | ||
Function-to-pointer conversion | Exact Match | ||
Qualification conversions | |||
Function pointer conversion | Qualification Adjustment | ||
Integral promotions | |||
Floating-point promotion | Promotion | Promotion | |
Integral conversions | |||
Floating-point conversions | |||
Floating-integral conversions | |||
Pointer conversions | Conversion | Conversion | |
Pointer to member conversions | |||
Boolean conversions |
struct A {};
struct B : public A {} b;
int f(A&);
int f(B&);
int i = f(b); // calls f(B&), an exact match, rather than f(A&), a conversion
— end example
void f(std::initializer_list<int>);
f( {} ); // OK: f(initializer_list<int>) identity conversion
f( {1,2,3} ); // OK: f(initializer_list<int>) identity conversion
f( {'a','b'} ); // OK: f(initializer_list<int>) integral promotion
f( {1.0} ); // error: narrowing
struct A {
A(std::initializer_list<double>); // #1
A(std::initializer_list<complex<double>>); // #2
A(std::initializer_list<std::string>); // #3
};
A a{ 1.0,2.0 }; // OK, uses #1
void g(A);
g({ "foo", "bar" }); // OK, uses #3
typedef int IA[3];
void h(const IA&);
h({ 1, 2, 3 }); // OK: identity conversion
— end example
struct A {
A(std::initializer_list<int>);
};
void f(A);
f( {'a', 'b'} ); // OK: f(A(std::initializer_list<int>)) user-defined conversion
struct B {
B(int, double);
};
void g(B);
g( {'a', 'b'} ); // OK: g(B(int, double)) user-defined conversion
g( {1.0, 1.0} ); // error: narrowing
void f(B);
f( {'a', 'b'} ); // error: ambiguous f(A) or f(B)
struct C {
C(std::string);
};
void h(C);
h({"foo"}); // OK: h(C(std::string("foo")))
struct D {
D(A, C);
};
void i(D);
i({ {1,2}, {"bar"} }); // OK: i(D(A(std::initializer_list<int>{1,2}), C(std::string("bar"))))
— end example
struct A {
int m1;
double m2;
};
void f(A);
f( {'a', 'b'} ); // OK: f(A(int,double)) user-defined conversion
f( {1.0} ); // error: narrowing
— end example
struct A {
int m1;
double m2;
};
void f(const A&);
f( {'a', 'b'} ); // OK: f(A(int,double)) user-defined conversion
f( {1.0} ); // error: narrowing
void g(const double &);
g({1}); // same conversion as int to double
— end example
void f(int);
f( {'a'} ); // OK: same conversion as char to int
f( {1.0} ); // error: narrowing
— end example